1) Technical field of the Invention
The present invention relates to a semiconductor optical device, and in particular, relates to a semiconductor optical device used for an optical communication system and an optical disc device.
2) Description of Related Arts
Referring to FIGS. 21 and 22, a semiconductor optical device as denoted by numerical reference 500 according to the prior art will be described hereinafter. FIG. 21 is a perspective view of a λ/4-shifted distributed-feedback semiconductor laser device 500 as an example of the semiconductor optical device, and FIG. 22 is a cross sectional view taken along the line of XXII—XXII of FIG. 21.
The distributed-feedback semiconductor laser device 500 includes, in general, a buffer layer 2, and an active layer 3 subsequently formed on a substrate 1. The active layer 3 is a multiple quantum well (MQW) layer. Also provided on the active layer 3 are cladding layers 4 and 8. The active layer 3 is sandwiched between the buffer layer 2 and the cladding layer 4.
Also, grown adjacent to both end surfaces are p-type blocking layers 5, 7 and an n-type blocking layer 6 of sandwiched therebetween.
A contact layer 9, an insulating layer 10, and a cathode electrode 11 are formed in order on the cladding layer 8. Also, formed on the bottom surface of the substrate 1 are anode electrodes 12, 13.
Further, a plurality of beam guiding layers 14 are embedded within the cladding layer 4, performing as diffraction gratings. A phase shifting region 15 of the diffraction gratings is provided adjacent to a middle point between both end surfaces.
In the distributed-feedback semiconductor laser device 500, as illustrated in FIG. 22, a plurality of the diffraction gratings arranged with a predetermined space to each other cause the beams having a certain wavelength to reflect and resonate so as to generate the laser oscillation. Also, the phase shifting region 15 is adapted for shifting the phase of the beam by λ/4 (λ: wavelength) The active layer 3 in cooperation with the cladding layers 2, 4 sandwiching thereof defines a beam waveguide, and an oscillation structure is realized between the front- and rear-end surfaces.
According to the conventional distributed-feedback semiconductor laser device 500, the photon density of the active layer 3 becomes greater with distance from both end surfaces toward the phase shifting region 15 adjacent to the middle point between both end surfaces. Thus, the carrier density is reduced in the region adjacent to the phase shifting region 15 (hole burning effect). The reduced carrier density causes the plasma effect of the carriers to be decreased, increasing the refractive index of the active layer 3. Because of this result, when the variation of the refractive index across the oscillator (active layer 3) is too broad, the transverse mode of the laser oscillation, influenced by the difference of refractive index between the core portion and the cladding portion of the beam waveguide, becomes unstable. Thus, the linearity between current and optical output is adversely affected and causes a so-called kink. Therefore, the conventional distributed-feedback semiconductor laser device has a drawback in that it cannot achieve a high optical performance because of the insufficient optical output and the kink.
Other types of semiconductor laser devices such as a Fabry-Perot semiconductor laser device and a partial diffraction gratings semiconductor laser device also have the similar drawbacks as well.
Therefore, one of the embodiments of the present invention has a purpose to provide a kink-free semiconductor optical device stabilizing a laser oscillation and obtaining a high optical performance.